RFR 8199420 Update javadoc tags in java.lang.System and related

[Roger Riggs]

Please review some javadoc tag cleanup in java.lang.System, Threads, etc.
The changes are using {@code } instead of <code></code>,  @throws 
instead of @exception
and the removal of a few unnecessary <p> tags.

The are more many more files but these bugged me recently and I didn't 
want the changeset to be too large.

Webrev:
http://cr.openjdk.java.net/~rriggs/webrev-system-javadoc/index.html

Thanks, Roger